The use of epoxy resins to cure liquid carboxyl-containing polymers is known. This cure can be accelerated by the use of catalysts such as amines. However, a curable composition designed around molar equivalents of carboxyl and epoxide yields a weak, gummy vulcanizate. To obtain stronger cures, a molar excess of epoxide can be used. For example, U.S. Pat. No. 3,285,949 shows that a carboxyl-terminated polybutadiene rubber cured using a small molar excess of epoxy resin can yield vulcanizates having tensile strengths up to about 500 psi and elongations of 500 to 700%. Unfortunately, these vulcanizate properties are not sufficient for many applications such as molded auto parts and accessories. Further increasing the molar ratio of epoxy resin to liquid carboxyl-terminated polymer does increase the tensile strength of the vulcanizate. However, in obtaining increased tensile strength, elastomeric properties are sacrificed and the vulcanizate is more like a hard rubber or plastic than an elastomer. For example, a mix of 100 parts by weight of a diepoxy resin and 100 parts by weight of a carboxyl-terminated polymer, cured using an amine catalyst, yields a vulcanizate having up to 2000 psi tensile, but it will also have low elongation (rarely exceeding 100 percent), a high hardness, and a high Gehman Freeze point.
Simply adding reinforcing ingredients to a liquid polymer vulcanizable composition does not supply a totally satisfactory solution. Tensile strength can be increased with the addition of carbon black (see Liquid Butadiene/Acrylonitrile Polymers with Reactive Terminals by Drake and McCarthy, Rubber World, Oct., 1968; where ultimate tensile strengths of 1000 to 1500 psi were obtained without significant loss of rubbery properties). Unfortunately, a polymer mix containing sufficient carbon black (or any filler) to significantly improve the tensile is extremely viscous, resembling a thick putty. The vulcanizable polymer mix is not fluid and is not castable. Hence, one of the major advantages of using liquid polymers (i.e. their ability to flow in place prior to cure) is lost.